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Geometry Difficulty 6.6 National Olympiad Prove it Slovenia

The circles K1K_1 and K2K_2 intersect in two distinct points AA and BB. The tangents to K1K_1 through AA and BB intersect at TT. Let MM be an arbitrary point on the circle K1K_1 distinct from AA and BB. The line MTMT meets the circle K1K_1 again at CC, the line MAMA meets the circle K2K_2 again at KK and the line ACAC meets the circle K2K_2 again at LL. Prove that the line MCMC contains the midpoint of the segment KLKL.

Solution

Let PP be the intersection of the lines MTMT and KLKL.
By Menelaus' theorem for the triangle ALKALK and the colinear points C,PC, P and MM we have
ACCLLPPKKMMA=1. \frac{AC}{CL} \cdot \frac{LP}{PK} \cdot \frac{KM}{MA} = -1.
In order to show that PL=PK|PL| = |PK| it suffices to see that
ACCL=MAMK.(12) \frac{|AC|}{|CL|} = \frac{|MA|}{|MK|}. \qquad (12)
(Note: We will be using directed angles from this point on.)
The points B,C,AB, C, A and MM are concyclic, so BCL=ACB=BMK\nparallel BCL = -\nparallel ACB = \nparallel BMK. The points B,L,KB, L, K and AA are also concyclic, so CLB=ALB=MKB\nparallel CLB = \nparallel ALB = \nparallel MKB. The triangles BCLBCL and BMKBMK have two angles in common, so they are similar. It follows that
CLMK=BCBM.(13) \frac{|CL|}{|MK|} = \frac{|BC|}{|BM|}. \qquad (13)
By the tangent-chord angle theorem we have TBC=BMC\nparallel TBC = \nparallel BMC. The triangles TBCTBC and TMBTMB have two angles in common and are similar. Using the tangent-chord angle theorem again we see that CAT=CMA\nparallel CAT = \nparallel CMA. So, the triangles TACTAC and TMATMA have two angles in common and are similar. From here we find the ratios BCBM=TBTM\frac{|BC|}{|BM|} = \frac{|TB|}{|TM|} and ACAM=TATM\frac{|AC|}{|AM|} = \frac{|TA|}{|TM|}. Combining the two with the equality between the tangent segments TA=TB|TA| = |TB| we get
BCBM=ACAM.(14) \frac{|BC|}{|BM|} = \frac{|AC|}{|AM|}. \qquad (14)

(Note: The property (14) or AMBC=ACBM|AM| \cdot |BC| = |AC| \cdot |BM| is called the harmonic property of the cyclic quadrilateral.)
If we now combine the equalities (13) and (14), we get (12), which was our goal. From here we conclude that PP is the midpoint of KLKL and so the midpoint of KLKL does indeed lie on MCMC.

Figure 1

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